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To connect certain types of backend to your Stores, you can make use of fritz2’s repositories. fritz2 offers implementations of the repository-interfaces for two types of backends:

  • LocalStorage
  • REST
  • more like GraphQL are yet to come

The following examples use this Person data class:

data class Person(
    val _id: String = "",
    val name: String,
    val age: Int,
    val salary: Int


When defining the data class you want to share using a Repository, you must define a Resource for it to define, how to derive a unique id from an instance of your data class and how your data class can be (de-)serialized. You can of course use kotlinx-serialization for this. Annotate your data class (here Person) with @Serializable and then write the following serializer next to your class (in commonMain section):

object PersonResource : Resource<Person, String> {
    override val idProvider: IdProvider<Person, String> = Person::_id

    override fun deserialize(source: String): Person = Json.decodeFromString(Person.serializer(), source)
    override fun serialize(item: Person): String = Json.encodeToString(Person.serializer(), item)
    // needed when using QueryRepository otherwise you can omit
    override fun deserializeList(source: String): List<Person> = Json.decodeFromString(ListSerializer(Person.serializer()), source)
    override fun serializeList(items: List<Person>): String = Json.encodeToString(ListSerializer(Person.serializer()), items)


We differentiate two kinds of repositories both using ids of type I:

  • an EntityRepository deals with one single entity of a given type T. It offers the usual CRUD-methods:
    • load(id: I): T
    • addOrUpdate(entity: T): T
    • delete(entity: T)
  • a QueryRepository deals with a List of instances of a given type T and a query object Q. It offers the following methods to query or manipulate the content of the repository:
    • query(query: Q): List<T>
    • updateMany(entities: List<T>, entitiesToUpdate: List<T>): List<T>
    • addOrUpdate(entities: List<T>, entity: T): List<T>
    • delete(entities: List<T>, id: I): List<T>
    • delete(entities: List<T>, ids: List<I>): List<T>


To connect a Store with a REST-backend for example, just add the repository-service and use its methods in your handler:

// use the defined resource from above
val entityStore = object : RootStore<Person>(Person(...)) {

    private val rest = restEntity(PersonResource, http("https://your_api_endpoint"), initialId = "")

    val load = handle<String> { _, id ->

    val addOrUpdate = handle { rest.addOrUpdate(it) }

    val delete = handle { rest.delete(it) }
    init {

The restEntity function needs an initialId to distinguish add from update operations when calling addOrUpdate: A POSTrequest is sent, when the id of the given instance equals initialID, otherwise a PUT request is used to update the ressource.).

By calling syncBy from your init-block the given Handler (here addOrUpdate) is automatically called on each update of your Stores data, to keep your REST-backend in sync with your local ressource.


When creating a QueryRepository, you can define a type describing the queries which are done by this repository. You also have to implement a lambda, that defines, how to deal with a concrete instance of this query type:

data class PersonQuery(val namePrefix: String? = null)

val queryStore = object : RootStore<List<Person>>(emptyList()) {
    val localStorage = localStorageQuery<Person, String, PersonQuery>(PersonResource, "your prefix") { entities, query ->
      if (query.namePrefix != null) entities.filter { }
      else entities
    val query = handle<PersonQuery> { _, query ->
    val delete = handle<String> { list, id ->
      localStorage.delete(list, id)
    init {

Of course the receiver and result of this function depend on the concrete implementation you use. For a RestQuery, you have to build and fire the request according to your query-object. If you do not specify a function, all entities of the defined Resource will be returned.

val restQuery = restQuery<Person, String, PersonQuery>(PersonResource, "https://your_api_endpoint", initialId = "")

For more information about the remote parameter of restQuery, please refer to the docs section HTTP-Calls.


You can see repositories of all types in action at

Distributed by a MIT license.